If you were in a smooth-riding train with no windows, could you sense the difference between uniform motion and rest or between accelerated motion and rest? 1. Both acclerated and uniform motion can be sensed.
2. Only uniform motion can be sensed.
3. Only accelerated motion can be sensed.
4. No motion can be sensed.

3. Only accelerated motion can be sensed

Explanation:

Without windows on such a train, you'd have no frame of reference for your speed. By that I mean, without being able to see how fast you are moving past other things, it's almost as if you aren't moving at all... almost.

At rest you obviously aren't moving and in uniform motion, with a constant speed, it would feel as though you aren't moving. But during periods of acceleration you'll feel the force on your body (F=ma) and would be able to tell if you were moving in a particular direction.

You've probably felt this before. Maybe not on a windowless train but perhaps in a car or on a roller coaster. Speeding up makes you go back into your seat a bit and slowing down makes you lean forward a bit. Both speeding up and slowing down are examples of acceleration (just in different directions) and how fast you accelerate will affect how much force you experience.

So the answer would be option 3.

Side note: If the train wasn't smooth riding then there would be some amount of friction going on and you could probably tell if you were in motion by the products of that friction (like sound and vibrations) even at a constant speed.

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